1. Field of the Invention
The present invention relates to timing recovery methods and apparatuses in communication systems. More specifically, the present invention relates to methods and corresponding apparatus for timing recovery based on dispersion in the received signal. Software and components for implementing the novel methods according to the present invention are also disclosed.
The instant patent application is based on Provisional Patent Application No. 60/241,274 of Oct. 17, 2000, which application is incorporated, in its entirety, by reference.
2. Description of the Related Art
In any communication system, it is necessary to synchronize the clock at the receiver with the clock at the transmitter, a process that is commonly called synchronization or timing recovery. In other words, timing recovery is the derivation of a timing signal from the received signal. Timing recovery can be accomplished by a variety of methods, two of which are discussed below.
Recent efforts in developing digital radio/video broadcasting systems, e.g., HDTV broadcasting systems, have highlighted the problem of optimal timing recovery as a significant issue in digital receiver design. More specifically, one of the features of digital media broadcasting channels is long delay spread multipath; the traditional approach to this problem, i.e., inclusion of fractionally spaced equalizers that are insensitive to timing phase in each receiver, is impractical in consumer electronics receivers and the like.
The problem of timing recovery consists of estimation of the timing frequency and the optimal timing phase. It will be appreciated that there are many methods of attacking this problem; for instance, the standard text entitled “Communication Systems Engineering,” by Proakis et al. (Prentice Hall, N.J., 1994) describes several such methods. Circuitry for carrying of several of these methods will be discussed immediately below. Conventional timing recovery techniques and methods include the Early-Late Gate method, the minimum Mean-Squared-Error (MSE) method, the Maximum-Likelihood (ML) method, and the Output Energy Maximization (OEM) method. Both the Early-Late Gate and the ML methods were developed based on the assumption that there are no channel dynamics; the validity and performance of these methods are difficult to ascertain in the presence of several additional nontrivial channels, i.e., a multipath channel. On the other hand, the MSE criterion can be extended to the case where a multipath channel is present; however, due to the nature of the MSE algorithm, either a training sequence or feedback from the decision device must be employed in any practical implementation of the MSE algorithm. Since training sequences reduce the overall system throughput, and since the feedback from a decision device may be unreliable at the synchronization stage, a non-decision-directed (blind) method would be advantageous in time recovery in the presence of a multipath channel.
The output energy maximization method, which can be performed blindly, has been analyzed by D. N. Godard and reported in his article entitled “Passband Timing Recovery in All-Digital Modem Receiver” (IEEE Trans. Communications, Vol. 26, No. 5 (May 1978)). Another blind method which has not received significant attention utilizes the technique of dispersion minimization; this method is frequently referred to as employing a Constant Modulus Algorithm. This method can be implemented without a training signal. Guglielmi et al. considered the constant modulus approach to jointly optimize the combiner weights and timing offsets of a pair of received signals from two antennas. See Guglielmi et al., “Joint Clock Recovery and Baseband Combining for the Diversity Radio Channel,” IEEE Trans. Communications, Vol. 44, pp. 114–117 (Jan. 1996). However, the application of the constant modulus algorithm (or the minimization of the dispersion of the received signal) heretofore has not been applied, or even proposed, as a solution for timing phase recovery for a single antenna structure subject to substantial multipath.
It should be noted here that all of the publications mentioned above are incorporated herein by reference.
FIGS. 1A, 1B and 1C illustrate three alternative circuits which can be employed for timing recovery. The circuitry of FIG. 1A, for example, includes a signal path consisting of an analog processor 10a, a sample 20a, and a digital processor 30a. The circuit also includes a voltage controlled oscillator (VCO) 40a, which controls the sampler 20a by specifying when samples of the incoming signal are to be taken. It will be appreciated from FIG. 1A that the analog processor 10a, by controlling the rate of frequency of VCO 40a, indirectly determines when the sampling instants or events will occur. In contrast, the circuit illustrated in FIG. 1B employs the digital post processor 30b, rather that analog processor 10b, to control the rate of frequency of the VCO 40b and, hence, in determining when sampling events are to occur, i.e, when sampler 20b is to be operated. In contrast to both of the figures previously discussed, in the circuit of FIG. 1C the sampling instants at sample 20c are chosen based, not on using analog processor 10c or digital processor 30c, but rather on a free running clock 50; digital post processing is employed in recovering the values of the received signal that would have occur at the optical setting instants. None of these circuits are explicitly configured for blind timing recovery.
What is needed is a method and corresponding apparatus for timing recovery that can be performed “blindly” (without a training signal). Moreover, what is needed is a method and corresponding apparatus which can be readily implemented as either an analog procedure or in digital form. It would be advantageous if the method and corresponding apparatus were robust, e.g., insensitive to clock jitter and to the effects of intersymbol interference. What is also needed is a method which advantageously can be implemented in any of the three circuit variations illustrated in FIGS. 1A–1C.